Can You Read a Quake?

An earthquake has just hit. Can you tell where? You first need to know how to read a
seismogram.

To do this activity...
You should review how a seismograph works before you do this activity.

A good seismograph can detect seismic waves from an earthquake thousands of
kilometers away. In this activity, you will learn how to understand and get information
from a seismogram. When you know how to read a seismogram, you can determine
how strong the waves are (their intensity, amplitude, and energy). You can also
determine how far away the epicenter is from your seismograph. You will later learn
how to find this epicenter distance when you do the activity Race of the Waves.

Suppose your seismograph suddenly begins to receive earthquake waves. At the
same moment the first waves arrive, you start your stopwatch. This is how each city
below measured the waves that they received: They started their stopwatches when
the fist seismic wave (Primary wave) arrived. All of these waves in all of the cities
below came from the same epicenter. (Later you can locate this epicenter in the
activity Where Did the Quake Hit?).

1.Look carefully over the eight seismograms from the seismic stations around the
world.

a) Which location experienced the greatest shock? How can you tell?

b) Which location is probably closest to the epicenter?

c) Which location is the farthest from the epicenter?

2. A typical earthquake sends out at least three types of waves: Primary (P),
Secondary (S), and Surface (L) waves. These waves travel differently in the earth
and make different patterns on the seismogram. For example, L waves are normally
the last to arrive (take the longest time), but they also cause the most surface damage
to a city.

a) Tokyo received all three types of waves. Locate on the Tokyo
seismogram
the beginning of the P waves.
How many minutes did the Tokyo clock read when the first P waves hit Tokyo?

b) How many minutes did the Tokyo clock read when the first S wave hit
Tokyo?

c) How many minutes did the Tokyo clock read when the first L wave hit
Tokyo?

3.Name the four locations that received all three types of waves (P, S, and L).

4. From the moment the first P wave arrived, the seismogram shows how many
minutes passed before the S waves arrived, followed later by the L waves.

a) Which type of wave is the fastest?

b) Which type of wave is the slowest?

c) At which location did the P and S waves arrive closer together --
San Francisco or Hawaii? What does this tell you about the location of the epicenter?

5. At some locations, not all of the three waves arrived.

a) Which locations appear to have received only P and L waves, but no S waves?

b) Which locations appear to have received only L waves?

6. Some locations that are close to each other did not receive the same waves.

a) Did San Francisco receive a P wave? did Los Angeles?

b) Compared to the size of the earth, these two cities are close to each
other.

Why do you think the P wave arrived in one of these cities but not in
the other?

Make a good hypothesis to explain this observation.

7. Look at the L waves for Kingston, for Jamaica and for Miami, Florida.

a) Which one -- Kingston or Miami -- is probably closer to the
epicenter?

How can you tell from the seismograms?

b) Did Kingston receive a P wave? did Miami receive a P wave?

c) These two cities are not very far from each other on the earth.

Why did the P wave arrive in one of these cities but not in the other?